1. Field of the Invention
This invention relates to a plastic-molded-type hollow semiconductor device and a process for manufacturing the same.
2. Description of the Related Art
FIG. 13 is a schematic diagram showing the construction of a conventional plastic-molded-type hollow semiconductor device. In the drawing, a semiconductor element 1 is mounted on a base 2 having a cavity 4. The semiconductor element 1 and the base 2 are electrically connected to each other through metal wiring 3. A cover 5 is bonded to the base 2 by a bonding material 6 to define a closed space therebetween. Further, external terminals 7 electrically connected to an external device (not shown) are provided on the base 2.
The conventional plastic-molded-type hollow semiconductor device, constructed as described above, has been manufactured in the following manner: the semiconductor element 1 is mounted inside the cavity 4 of the base 2 which has the external terminals 7. Then, the base 2 is electrically connected with the semiconductor element 1 by the metal wiring 3, which is usually made of a material mainly consisting of gold, aluminum or the like.
The semiconductor device, which is manufactured in the above-described manner, is plastic-molded to seal the semiconductor element 1 therein, thereby obtaining requisite reliability and handling properties. In the semiconductor device shown in FIG. 13, the cover 5 is mounted on the base 2 and bonded thereto so that a closed space is formed including the cavity 4.
Generally speaking, the cover 5 consists of an insulating material, such as alumins whose surface has been treated into alumite, or a ceramic material. The base 2 may be made of a ceramic material in order to obtain high reliability and airtightness, or of a plastic material, which is relatively inexpensive. The bonding material 6, which consists of resin, glass, metal, etc., is previously applied to the bonding positions on the cover 5. In the semiconductor device shown in FIG. 13, the cover 5 consists of a ceramic material (manufactured by Kyocera Corporation) as shown in FIG. 14.
In the above-described plastic-molded-type hollow semiconductor device, the bonding material is applied to the bonding positions on the cover 5 beforehand, and the cover 5 and the base 2 are bonded together after having been separately pre-heated. The pre-heating temperature is close to the melting point or curing temperature of the bonding material 6. The ambrent gas at the time of bonding is generally required to be an inert gas. However, air may also be used. The ambrent is also pre-heated to the same temperature as the base 2 and the cover 5.
In the bonding of the base 2 and the cover 5, a balance in the temperatures of the base 2, of the cover 5 and of the cavity 4 defined therebetween, is very important. When this balance cannot be maintained, a through-hole 8 is generated in the bonding material 6 as shown in FIG. 15, resulting in a deterioration in airtightness and reliability.
This phenomenon is attributable to the fact that the temperature of the inner gas trapped in the cavity 4 is lower than the bonding temperature of the base 2 and the cover 5, which are bonded together after having been separately pre-heated. Thus, the inner gas expands due to a temperature rise as a result of the bonding, thereby causing the pressure in the cavity, which has a fixed volume, to rise. As a result, the inner gas forces its way to the exterior, thereby causing the through-hole 8 to be generated. The tendency for this phenomenon to occur was more conspicuous when the bonding material was a thermosetting resin.
Further, the generation of the through-hole 8 not only is the main cause for a deterioration of the reliability of the semiconductor device but also leads to a reduced yield.